[21.05] Fix radvd config to use the correct interface name

[sysvinit]

The radvd configuration contains some hard-coded assumptions that downstream interfaces are always ethXXX, however since the move to VXLAN this is no longer correct. Use the interface name derived by the platform code directly. Additionally, use the static list of networks with floating gateways for generating the configuration instead of excluding an (outdated) list of networks from all which are available.

PL-133201

@flyingcircusio/release-managers

Release process

• Created changelog entry using ./changelog.sh

PR release workflow (internal)

• PR has internal ticket
• internal issue ID (PL-…) part of branch name
• internal issue ID mentioned in PR description text
• ticket is on Platform agile board
• ticket state set to Pull request ready
• if ticket is more urgent than within the next few days, directly contact a member of the Platform team

Design notes

• Provide a feature toggle if the change might need to be adjusted/reverted quickly depending on context. Consider whether the default should be on or off. Example: rate limiting.
• All customer-facing features and (NixOS) options need to be discoverable from documentation. Add or update relevant documentation such that hosted and guided customers can understand it as well.

Security implications

• Security requirements defined? (WHERE)
  • Correcting typos in configuration.
• Security requirements tested? (EVIDENCE)
  • Manually verified in DEV.